In order to operate a field device in an explosive atmosphere, various measures are necessary. These have the goal of preventing a spark formation, which, in given cases, could trigger an explosion, or avoiding that a spark occurring in the interior of a closed space has effects on the environment. In this regard, different protection regions are distinguished, and correspondingly there are different standards for different environmental conditions and applications. The standard DIN EN60079-11 defines the necessary safety requirements for device protection by intrinsic safety, known under the abbreviation Ex-i. This is an ignition protection type, which rests on the limiting of electrical energy within devices and/or connecting lines, which are exposed to a possibly explosive atmosphere, to a level below that at which an ignition can be brought about either by spark formation or by heat. Moreover, different protection levels are distinguished within this protection class, namely Ex-ia, Ex-ib and Ex-ic. These define the existing failure safety of the individual components and connecting lines.
For an explosion protection under class Ex-I, it must especially be assured that the electrical variables, electrical current, voltage and power, lie in a device at all times below respective, predetermined limit values. These limits values are so selected that, in the case of malfunction, e.g. because of a short circuit, the maximum occurring heat is not sufficient to produce an ignition spark. In this regard, the electrical current is frequently kept below its predetermined limit value by resistances, the voltage below its predetermined limit value by diodes, especially zener diodes, and the power below its predetermined limit value by a suitable combination of electrical current- and voltage limiting components. Such a protection circuit can be purchased, for example, under the designation MCR-PLUGTRAB PT from the firm, Phoenix Contact, as a pluggable apparatus. With reference to field devices, further, a number of examples are known, for example, in European Patent, EP0882955A1, German Patent, DE29809853U1 or published international application, WO2004/098014A1, in the case of which the requirements for explosion protection are fulfilled by particular circuit arrangements. WO2004/098014A1 describes arrangements of series resistors especially for the units for electrical current- and voltage limiting, while DE29809853U1 provides the application of Zener diodes. The application of diodes has, however, the disadvantage that the applications are limited to a certain temperature range.
Furthermore, the above mentioned examples of explosion protection circuits have no impedance matching. This concerns especially field devices, in the case of which the communication between electronics unit and sensor element occurs by means of alternating electrical current or alternating voltage. Typical frequencies lie in the range from 10 kHz to 50 MHz. The problem is that the load impedance of the sensor element is often not matched to the line, respectively electronics unit. This leads to standing waves on the line. Because of this, the complete power cannot be transmitted. Examples of such field devices include those where the sensor element drives or is driven by at least one piezoelement. These concern, among others, ultrasonic, flow measuring devices or vibronic, fill level measuring devices, such as sold by the applicant in great variety, in the first case under the designations, PROSONIC DDU10, PROSONIC Proline P, or PROSONIC Flow, and in the second case, under the designations, SOLIFANT and LIQUIFANT.
A typical method for assuring impedance matching, especially to a piezoelement, applies coils or transformers. The respective components must, based on their characteristics, be matched to the line, to the electronics unit, which has, in this case, usually an amplifier, and to the piezoelement.